Influenza, a vaccine-preventable disease, remains as an important cause of morbidity and mortality worldwide. All age groups are affected, but children experience the highest disease incidence while adults suffer the most serious disease complications and related mortality. The burden of seasonal influenza epidemics and the continuous threat of a potential pandemic influenza highlight the need for effective preventive strategies. During influenza epidemics, children are typically affected early and they serve as disease vectors, introducing influenza into the community. Influenza vaccines have proven to be efficacious in the prevention of disease in vaccinated individuals and it has been hypothesized that immunization of a significant proportion of children could have major beneficial effects in the community, through the interruption of influenza transmission. Therefore, school-based influenza immunization programs have been proposed as prevention models. Nevertheless, the effectiveness of these approaches has not been convincingly demonstrated in large scale interventions. If proven effective, school-based influenza immunizations could become one major strategy for prevention of both seasonal and pandemic influenza. In October 2005, a large school-based influenza immunization campaign was initiated in Knox County, TN. The campaign was successfully implemented, immunizing 24,281 (46%) school-aged children attending public schools in the county. The campaign achieved at least similar coverage in the subsequent 2006-2007 influenza season. Although this campaign was set up as a feasibility demonstration project, its impact on disease outcomes has not been examined. We hypothesize that this large 2-year intervention decreased the incidence of influenza related diseases in both vaccinated and unvaccinated Knox County residents. Vanderbilt investigators are currently performing active viral surveillance in Knox County (intervention) and Davidson County (control) to compare viral activity during the second year of the campaign (2006-2007 season). We propose to perform additional and complementary analyses using alternate data sources to measure the overall intervention effectiveness (direct and indirect effects) over both intervention years. We will utilize large electronic databases that systematically record healthcare encounters in the State of Tennessee, and the infrastructure and expertise of the New Vaccine Surveillance Network (NVSN), a prospective surveillance system for viral infections in children that is currently operating in Tennessee. The proposed research and career development plan complement Dr. Grijalva's training in Medicine, Public Health and Epidemiology. Furthermore, this proposal supports his efforts to develop the expertise needed to become an independent investigator focused on prevention of influenza-related morbidity and mortality and on vaccine program and policy evaluations.